Micromachined microphones typically include a thin diaphragm electrode and a fixed sensing electrode that is positioned alongside the diaphragm electrode. The diaphragm electrode and the fixed sensing electrode act like plates of a capacitor. During operation of the microphone, charges are placed on the diaphragm electrode and the fixed sensing electrode. As the diaphragm electrode vibrates in response to sound waves, the change in distance between the diaphragm electrode and the fixed sensing electrode results in capacitance changes that correspond to the sound waves.
FIG. 1 shows the general structure of a micromachined microphone as known in the art. Among other things, the micromachined microphone includes a diaphragm 102 and a bridge 104. The diaphragm 102 and the bridge 104 act as electrodes for a capacitive circuit. As shown, the bridge 104 may be perforated to allow sound waves to reach the diaphragm 102. Alternatively or additionally, sound waves can be made to reach the diaphragm through other channels. In any case, sound waves cause the diaphragm to vibrate, and the vibrations can be sensed as changes in capacitance between the diaphragm 102 and the bridge 104. The micromachined microphone typically includes a substantial cavity 106 behind the diaphragm 102 in order to allow the diaphragm 102 to move freely.
In a typical micromachined microphone, sound waves reach the diaphragm through perforations in the fixed sensing electrode. The size and depth of the perforations can affect the quality of sound reproduction.